1. Field of the Invention
The present invention relates to a fluid bearing apparatus having a uniform dynamic pressure distribution and, more particularly, to a fluid bearing apparatus having a uniform dynamic pressure distribution in which at least two bending parts are formed in order to generate fluid pressure to raise a rotary body from a supporting unit of the rotary body.
2. Description of the Related Art
Recently, as the computer-related industries have continuously been developed, driving motors for various kinds of facilities, such as a head driving apparatus for a video tape recorder (VTR), an optical polygon driving apparatus for a laser printer, a camcorder driving motor, etc., need to have a high density and a miniaturized form. Such driving apparatuses require a bearing which is precise and stable, and has a super high rotation performance. In compliance with such a requirement, a fluid bearing apparatus has been developed.
In a dynamic pressure fluid bearing there is commonly provided a fluid having a predetermined coefficient of viscosity in order to reduce the frictional force between a rotary body and a rotary body supporting unit which supports the thrust load of the rotary body. Dynamic pressure generating grooves are formed in the supporting unit of the rotary body to generate the dynamic pressure. By rotating the rotary body on the supporting unit of the rotary body with a predetermined speed, a predetermined pressure is generated between the rotary body and the supporting unit.
Generally, the dynamic pressure generating grooves have variously been developed. Out of the dynamic pressure generating grooves, herringbone-shaped grooves and spiral-shaped grooves are the most well known.
The operation of a head driving apparatus of a VTR equipped with the dynamic pressure fluid bearing apparatus applying one of the above-mentioned dynamic pressure generating grooves will be illustrated, with reference to FIG. 1.
As shown in the FIG. 1, the VTR head driving apparatus 20 includes: an upper drum 12 which is rotated and on which a head tip 11 for reading a video signal and an audio signal recorded on a VTR tape is mounted; a lower drum 13 which is fixed; a shaft 14 which is fixed to the lower drum 13 along the rotating center of the upper drum 12; a doughnut-shaped thrust bearing 15 which minimizes the friction between the upper drum 12 and the lower drum 13 caused by the thrust load of the upper drum 12; a stator 16 which is mounted to the shaft 14 but spaced apart from the center of the shaft 14; and a rotor 17 which is located on the upper drum 12 spaced from the stator 16 to generate the rotary power of the upper drum 12.
A plurality of herringbone-shaped dynamic pressure generating grooves are formed on the circumferential surface of the shaft 14, in order to support the radial load of the upper drum 12.
As the head tip 11 is located at the upper drum 12 of the VTR head driving apparatus 20 having the above-described structure, in the case where the head tip 11 is vibrated or oscillated, reproducing the video and audio signals becomes difficult and the state of storing the signals becomes bad. As a result, the role of the thrust bearing is very important.
Reference numerals 19a and 19 indicate a stator transformer and a rotary transformer, respectively.
FIG. 2 is a plan view illustrating the conventional thrust bearing 15. The thrust bearing 15 is a cylindrical doughnut shape having a predetermined height and an inside diameter to which the shaft 14 is tightly fitted. On the upper surface of the thrust bearing 15 where the thrust bearing 15 and the upper drum 12 contact, a plurality of dynamic pressure generating grooves 15a are formed.
Generally, as the thrust bearing 15 is repeatedly contacted/separated to/from the upper drum 12, it is usually made of a material which is resistant to abrasion.
The dynamic pressure generating groove 15a formed at the thrust bearing 15 is generally a cramped herringbone shape, and a plurality of the dynamic pressure generating grooves are arranged in a circle centering around the center of the thrust bearing 15 along the circumference at a predetermined interval.
The dynamic pressure generating groove 15a is etched at a predetermined depth by an etching process. When etching the plain surface, there are two wall sides and one base side. Out of the two wall sides, one is defined as a first side wall 15c and the other is defined as a second side wall 15b.
The dynamic pressure is generated by the first side wall 15c and the second side wall 15b, and this will now be explained in detail.
As mentioned above, as the thrust bearing 15 is a doughnut shape, it has an inner diameter and an outer diameter, which are referred to as B and A, respectively.
Moreover, a straight line from the center O of the thrust bearing 15 to the outer diameter A is defined as H. After that, an imaginary concentric circle D between the outer diameter A and the inner diameter B is determined.
At this time, as the size and the position of the dynamic pressure by the concentric circle D, the concentric circle D is determined based on the size of the load on the upper drum 12 of the VTR head driving apparatus.
When the concentric circle D is determined, a point h where D and H meet is determined. When the point h is determined, each straight line is drawn to the outer diameter A and inner diameter B from the point h, and thereby the points d and c are determined. The angle between d and c is .theta.2. A line /chd formed by connecting the points c, h, and d becomes the second side wall 15b.
Moreover, a straight line H' is formed by rotating the straight line H based on the center of the thrust bearing 15 by a predetermined angle. The rotation angle relates to the dynamic pressure generating area, and the dynamic pressure generating area closely relates to the size of the dynamic pressure.
A point where H' and the concentric circle D meet is defined as g. From the point g, each straight line is drawn to the outer diameter A and inner diameter B, and thereby points a and b are determined. The angle between a and b is .theta.1. A line /agb formed by connecting the points a, g, and b becomes the first side wall 15c.
In the dynamic pressure generating groove 15a having the above-described structure, when the upper drum 12 is rotated after the power is applied to the stator 16 and the rotor 17, the fluid flows into the first and second side walls 15c and 15b of the dynamic pressure generating groove 15a due to the boundary friction of the fluid located between the bottom of the upper drum 12 and the upper surface of the thrust bearing 15.
The distribution of the fluid pressure is explained, referring to a graph shown in FIG. 2. At the parts A and B which are the outer diameter and inner diameter of the thrust bearing 15, the fluid pressure is the lowest. As the fluid flows into the concentric circle D from the parts A and B, the fluid pressure gradually increases. This is because the fluid flowing in from both parts A and B is gathered in the part D. At this time, a peak fluid pressure Pmax is generated at the part D.
When the peak fluid pressure becomes larger than the load of the upper drum 12, the upper drum 12 is apart from the thrust bearing 15 and it is rotated without contacting each other.
However, as the conventional bearing 15 has a single bending part h and g at the dynamic pressure generating groove 15a in order to rotate the upper drum 12 which is the rotary body with a minimum frictional force, a single peak fluid pressure is generated.
Accordingly, when the direction of the load of the upper drum 12 varies or an impact is applied from the outside, the rotational stability of the upper drum 12 decreases and vibration and oscillation can be generated.